U.S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a tandem type of direct printing apparatus. In the direct printing apparatus, four printing stations are disposed along a sheet conveying direction. Each printing station comprises a toner carrier retaining toner on its outer periphery, a backing electrode opposed to the toner carrier and a printing head disposed between the toner carrier and the backing electrode, the printing head having a plurality of apertures and a plurality of electrodes surrounding each aperture. On the outer periphery of the toner carrier in each printing station are retained toner having different colors, for example, magenta, cyan, yellow and black. The backing electrode of each printing station is electrically connected to a power source, thereby between the toner carrier and the backing electrode is formed an electric field for attracting the toner on the toner carrier and propelling it toward the backing electrode through the apertures of the printing head. Between the printing head and the backing electrode in each printing station is formed a passage for a sheet.
When an ON voltage is applied to the electrode of the printing head in the printing station positioned at the most upstream side in the sheet conveying direction, for example, the magenta printing station, the toner attracting force due to the electric field between the toner carrier and the backing electrode propels the toner on the toner carrier through the apertures toward the backing electrode and adheres it to the sheet. When an OFF voltage is applied to the electrode of the printing head, the toner attracting force does not affect the toner on the toner carrier, whereby the toner is never propelled. Thus, when ON and OFF voltage applied to the electrode of the printing head are controlled on the basis of a desired image signal, a magenta image corresponding to the image signal is printed on the sheet. In the same manner, by controlling the ON and OFF voltage applied to the electrode of the printing head in each of the downstream printing stations a different color of image is laid on the previously printed image to form a desired image.
In the aforementioned tandem type of direct printing apparatus, since the different color of the toner is retained on the toner carrier of each printing station, the electric charge quantity of the toner is different at each printing station, causing variance of the toner retaining force on the surface of the toner carrier. Moreover, the printing station at the most upstream side in the sheet conveying direction performs print on the sheet surface where the toner is not adhered yet, while the printing station at the downstream side performs print again on the toner adhered to the sheet surface by the upstream printing station. Thus, the laminated condition of the toner is different at each printing station, causing variance of the attracting force due to the electric field between the toner carrier and the backing electrode. The variance of both the toner retaining force and the attracting force also causes variance of characteristic that toner is detached from the toner carrier and propelled by the attracting force due to the electric field between the toner carrier and the backing electrode, i.e. transferability at each printing station, resulting in a disadvantage that desirable image is difficult to obtain.